Research Of Leakage Flow And Near Stall Characteristics In Mixed-flow Pump

Mixed-flow pump has many advantages,such as large flow rate,wide efficient area and strong cavitation resistance.It is widely used in water conservancy,farmland,municipal engineering,aerospace and other fields,and plays an important role in national economic construction.During the operation of mixed-flow pump,because of the clearance between the impeller rim and the runner chamber,the leakage flow of the rim will occur.The leakage flow interferes with the mainstream motion and mixes with the mainstream to form rim leakage vortices,which can easily induce flow instability and affect the stability of the pump.At the same time,the mixed-flow pump is prone to rotating stall under small flow conditions,which will not only cause the performance of the mixed-flow pump to decline,but also cause fluid excitation in serious cases,resulting in potential safety hazards of the unit.It is found that there is a close relationship between the rim leakage flow and the rotating stall phenomena when the flow instability occurs.Therefore,the study of rim leakage flow and the near stall characteristics of mixed-flow pump is of great significance to explore the mechanism of flow instability,reveal the internal relationship between leakage flow and rotating stall,and provide a new idea for restraining rotating stall phenomena.In this paper,a low specific speed guide vane mixed-flow pump is studied by changing the rim clearance,which is financed by the National Natural Fund Project"Dynamic Behavior and Instability Mechanism of Mixed-flow Pump Rim Leakage Flow under Near Stall Condition"(Project No.51679111).The main research work and achievements of this paper are as follows:(1)Three models of mixed-flow pump with rim clearance of 0.2mm,0.5mm and0.8mm were established respectively.Based on the numerical method,the external characteristics,leakage rate,the shape trajectory of leakage flow and vortex and the influence on the flow field of impeller inlet under different rim clearance were analyzed.The results show that there is a positive slope characteristic in the external characteristic curves under different clearances,and the operating point of stall is basically the same.The larger the clearance is,the larger the leakage rate is,and the change rule of leakage rate has a certain correspondence with the shape and trajectory of leakage flow.Under near stall condition,the flow field at the impeller inlet is greatly affected by leakage flow,but the influence mechanism of the flow field at the leading edge of adjacent blade is different from that at the leading edge of current blade.At the same time,the structure of the leakage vortex at the impeller outlet has changed greatly,which results in the blockage of the flow passage.And,the larger the clearance is,the larger the vortex is and the stronger the blocking effect is.(2)The near stall characteristics of mixed-flow pump with different rim clearances are studied,and the unsteady spatial structure and time characteristics of flange leakage flow and leakage vortex are analyzed.The results show that the stall core is located at the outlet of impeller and propagates from the leading edge of adjacent blades to the next passage,along the opposite direction of blade rotation.The pressure gradient and energy loss in stall flow passage are larger,which will also affect the guide vane flow passage.Whenδ>0.2mm,the number of stall core changes from one to two and in two adjacent channels.The propagation mechanism of stall core also tends to be complex.Whenδ=0.8mm,the propagation period decreases.When the flow passage changes from stall state to non-stall state,the leakage flow also changes from one state to another.Leakage flow can be used as a form of apparent stall,which better reflects the flow situation in the pump when rotating stall occurs.(3)The time domain and frequency domain characteristics of pressure fluctuation at different monitoring points at the stall core position of mixed-flow pump under different rim clearances are studied.The results show that the pressure fluctuation time domain curves between adjacent monitoring points show a large phase difference under near stall condition,which is consistent with the propagation period of stall core.There is a strong pressure drop on the time domain curve,which is the main propagation characteristic of the pressure fluctuation curve near stall.With the increase of clearance,the propagation mechanism of stall core tends to be complex,and the corresponding quadratic trough appears in a period of the pressure fluctuation time domain curve.Under near stall condition,the main frequency of the pressure fluctuation frequency domain curve at the monitoring point is 0.2f_n,i.e.stall frequency,and the main frequency amplitude of the frequency domain curve at monitoring point Y8 in the middle of the outlet is the largest.(4)The reliability of numerical simulation is verified by external characteristics and PIV tests.The influence of clearance size on the flow field in the dynamic and static interference zone of impeller and guide vane of mixed-flow pump is analyzed,and the characteristics of flow field under near stall condition are discussed.The results show that the head and efficiency of the numerical simulation are in good agreement with the experimental results,and the numerical simulation results of the flow fields with different phase sections are in good agreement with the experimental results,so the simulation results have high reliability.Under near stall condition,due to the influence of the rim leakage vortex at the upper blade outlet,the streamlines from the impeller outlet to the guide vane inlet at the blade inlet position show an angle of nearly 90degrees,and the larger the clearance,the smaller the angle.